1. Field of the Invention
This invention generally relates to fiber optic gyroscopes, and methods of making and operating fiber optic gyroscopes.
2. Description of the Related Art
Fiber-optic gyroscopes are included in a powerful class of sensors which bring to measurement systems many of the advantages that optical-fiber technology has brought to communications systems. For example, the very high bandwidth of optical fibers used in fiber optic gyroscopes allows the fiber optic gyroscope to convey a large amount of measurement information through a single fiber. In addition, because optical fiber is a dielectric, it is not subject to interference from electromagnetic waves that might be present in the sensing environment. Furthermore, fiber-optic gyroscopes typically can function under adverse conditions of temperature and pressure, and toxic or corrosive atmospheres that generally erode metals at a rapid rate.
The problem inherent in many conventional fiber optic gyroscopes, however, is that they can be sensitive to excess noise disturbances at low rotation rates. For example, the well known Raleigh scattering (i.e., scattering of light due to inhomogeneities in material density smaller than a wavelength in size), polarization noise (i.e., polarization fluctuations observed via voltage fluctuations), and zero rotation drift due to the Kerr effect (i.e., the development of birefringence when an isotropic transparent substance is placed in an electrical field) are typical problems which often reduce the accuracy of the optical gyroscope output by introducing errors in rotation rate sensing.
To minimize the errors in rotation rate sensing resulting from excess noise disturbances, fiber optic gyroscope system designers typically use broadband optical sources in gyroscope system construction. More particularly, fiber optic gyroscope designers typically use broadband optical sources with a stable spectra, such as, for example, super luminescent diodes (SLDs) or super luminescent fiber sources (SLSs), etc. The downside to using these sources, however, is that due to their finite bandwidth, these broadband sources introduce an additional excess noise term into the gyro output. This, in turn, causes a reduction in performance and satisfaction of the fiber optic gyroscope systems. It is, therefore, desirable to eliminate the excess noise component introduced by the broadband source in the gyro output to achieve optimum gyroscope performance.
Unfortunately, where SLDs are implemented in fiber optic gyroscopes, the fiber optic gyroscope generally suffers from a high wavelength sensitivity to temperature, inefficient coupling to single-mode fibers, and a lack of immunity to optical feedback. Consequently, in recent years, fiber optic gyroscope designers have focused less on fiber optic gyroscope systems using SLDs and more on super luminescent fiber sources (SLSs), which do not typically exhibit most of the problems inherent in the SLDs.
For example, where SLSs are used the fiber optic gyroscope will have a more stable response over temperature ranges inside the SLS spectrum. That is, the temperature stability of the SLS spectrum (in particular, its center wavelength) is far superior to that of the SLDs, whose emission wavelength typically varies by about 0.05 nm/deg C. Furthermore, by designing with SLSs, the fiber optic gyroscope designer is capable of generating more power in an SLS than is available in an SLD. For example, in a typical SLD, the available power is approximately 30 mW, of which probably no more than a few milliwatts can be coupled to a single mode fiber. On the other hand, where a typical SLS is used, the fiber optic gyroscope designer is capable of generating approximately 40 mW to 200 mW of power. Additionally, in a practical system, unwanted spurious reflections from the source/system interface can greatly reduce the power which can be coupled to the system fiber. These reflections can be minimized in the SLS fiber device by splicing the source and system fibers with a fused glass-to-glass splice, which typically can not be realized with SLDs. Finally, the high conversion efficiency of the SLS fiber source and its broad character pump band make SLSs a beneficial choice over SLDs for many compact, laser-diode-pumped configurations.
Super luminescent fiber sources (SLSs) typically consist of a single-mode fiber with a core doped with an ionictrivalent rare earth element, such as model HG980 from Lucent Technologies in Chesterfield, Mo., a pump laser such as FLD148G3NL-S from Fujitsu of Japan, and a wavelength division multiplexer (WDM) such as model WS1415-LW from JDS Uniphase in Bloomfield, Conn. SLS""s are well known in the art, and have been advantageously used to provide broadband (e.g., on the order of 10-30 nanometers) laser-like (highly directional) light beams for multiple applications, particularly in the communications field. For a description of an exemplary super luminescent fiber source, see xe2x80x9cAmplification of Spontaneous Emission in Erbium-Doped Single-Mode Fibers,xe2x80x9d by Emmanuel Desuvrie and J. R. Simpson, published by IEEE, in xe2x80x9cJournal of Lightwave Technology,xe2x80x9d Vol. 7, No. 5, May 1989, incorporated herein by reference.
As noted, an SLS typically includes a length of single-mode fiber, with a core doped with an ionic trivalent rare earth element. For example, neodymium (Nd3+) and erbium (Er3+) are rare earth elements that may be used to dope the core of a single-mode fiber so that the core acts as a laser medium. During operation, the fiber receives a pump input signal at one end, which is provided by a pump laser. The pump input signal is typically a laser signal having a specific wavelength xcexp. The ions within the fiber core absorb the input laser radiation at wavelength xcexp so that electrons in the outer shells of these ions are excited to a higher energy state of the ions. When a sufficient pump power is input into the end of the fiber, a population inversion is created (i.e., more electrons within the ions are in the excited state than are in the ground state), and a significant amount of fluorescence builds up along the fiber in both directions. As is well known, the fluorescence (i.e., the emission of photons at a different wavelength xcexs) is due to the spontaneous return of electrons from the excited state to the ground state so that a photon at a wavelength xcexs is emitted during the transition from the excited state to the ground state. The light which is emitted at the wavelength xcexs from the fiber is highly directional light, as in conventional laser light. One main characteristic of this emission which makes it different from that of a traditional laser (i.e., one which incorporates an optical resonator), however, is that the spectral content of the light emitted from the super luminescent fiber sources is generally very broad (between 1 and 30 nanometers). Thus, the optical signal output by the fiber will typically be at wavelength xcexp +/xe2x88x92 about 15 nanometers.
The construction and operation of conventional fiber optic gyroscopes is well known, and as such, will not be discussed in detail. A typical discussion of fiber optic gyroscopes may be found in U.S. Pat. No. 5,465,149 issued Nov. 7, 1995 to Strandjord, et al., and incorporated by reference herein. For illustrative purposes, FIG. 1 illustrates an exemplary fiber optic gyroscope system 100, which may be found in the prior art. In general, the optical portion of the system 100 contains several features along the optical paths to assure that this system is reciprocal. That is, when considering the system, substantially identical optical paths occur for each of the opposite direction propagating electromagnetic waves except for the specific introductions of non-reciprocal phase difference shifts, as is described below. In general, the features along the optical paths include a fiber optic light source 112, a fiber (tap) coupler 116, a multifunctional processing chip (e.g., integrated optics chip) 120, and a fiber optics coil 110, which are all variously connected by optical fiber portions 114, 118, 124 and 126.
Coiled optical fiber forms the coil 110 about a core or spool using a single mode optical fiber wrapped about the axis around which rotation is to be sensed. The use of a single mode fiber allows the paths of the electromagnetic or light waves to be defined uniquely, and further allows the phase fronts of such a guided wave to also be defined uniquely. This greatly aids maintaining reciprocity.
Light source 112 may be any broadband light source for propagating electromagnetic waves through the fiber optics system 100. This source 112 is typically a semiconductor super luminescent diode or a rare earth doped fiber light source which provides electromagnetic waves near the infrared part of the spectrum, over a range of typical wavelengths between 830 nanometers (nm) and 1550 nm. In general, source 112 will have a short coherence length for emitted light to reduce the phase shift difference errors between these waves due to Rayleigh and Fresnel scattering at scattering sites in coil 110.
Between light source 112 and fiber optic coil 110 is an optical path arrangement formed by the extension of the ends of the optical fiber forming coil 110 to some optical coupling components which separate the overall optical path into several optical path portions. As shown, optical fiber portion 114 is positioned against light source 112 at a point of optimum light emission from it, and, additionally, extends to the optical directional coupler 116 (also referred to as a optical light beam coupler or wave combiner and splitter), in which the optical fiber portion 114 ensures that the light source 112 and coupler 116 are in constant communication.
Coupler 116 has light transmission media inside which extend between four ports a, b, c, and d, which are shown on each end of coupler 116. Port a is connected to light source 112 via optical fiber 114 positioned against it. At port b, on the sense end of optical directional coupler 116 is a further optical fiber 136 which extends to be positioned against a photodetector 138.
Photodetector 138 detects electromagnetic waves, or light waves, impinging on it from optical fiber portion 136 positioned against it and provides a photo current in response to a signal component selector (not shown). This photocurrent, as indicated above, in the case of two nearly coherent impinging light waves, follows a raised cosine function in providing a photocurrent output which depends on the cosine of the phase difference between such a pair of substantially coherent light waves. This photodetector device will operate at a very low impedance to provide the photo current which is a linear function of the impinging radiation, and may typically be a p-i-n photodiode.
Also positioned against coupler 116 is an optical fiber 134, which may typically not be used in the operation of the gyroscope. Abutting against port c of coupler 116 is yet another optical fiber 118 extending to multifunctional integrated optics chip 120, including a phase modulator 128, and integrated optics waveguides 122 and 130 which form a y-junction 132. Leading from multifunctional processing chip 120 are optical fibers 124 and 126, which are connected to fiber coil 110 via waveguides 122 and 130 respectively.
Between port b of fiber coupler 116 and the gyroscope output are various photosensitive and electrical components designed to sense and generate an output corresponding to the rotational speed of fiber coil 110. This includes a photodetector 138, an analog signal conditioning device 140, an analog to digital converter (A/D) converter 142, a digital demodulator 144, and a square wave bias modulator 146, where each element is maintained in electrical communication during the processing of the fiber optic gyroscope system output. The function of each of the previously mentioned elements is well known in the art. Consequently, the elements are only briefly discussed below to aid in the understanding of the operation of the fiber optic gyroscope system 100.
Optical directional coupler (e.g., fiber coupler) 116, in receiving electromagnetic waves, or light, at any of its ports, transmits this light so that approximately half of the transmitted light appears at each of the two ports of the coupler 116 on its end opposite that end having the incoming port. On the other hand, no such waves or light is transmitted to the port which is on the same end of coupler 112 as is the incoming light port. For example, light received at port a will be transmitted to ports c and d, but not to port b. Similarly, light received at port c will be transmitted to ports a and b, but will not be transmitted to port d, and so on.
Therefore, during operation, light source 112 transmits a broadband light wave to port a of coupler 116 via optic fiber 114. Fiber coupler 116 splits the transmitted light and provides the light to ports c and d, where the light provided to port d typically may not be used by the gyroscope. The light provided to port c, however is further transmitted to multifunctional integrated optics chip 120 via optic fiber 118, where the light wave is further split at y-junction 132 and provided to waveguides 122 and 130.
The light provided to waveguide 122 is transmitted to fiber coil 110, via optic fiber 124, where it propagates clockwise around the length of fiber coil 110 (hereinafter, xe2x80x9cthe cw wavexe2x80x9d). Similarly, the light wave in waveguide 130 is provided to fiber coil 110 via optic fiber 126, where the light wave propagates counterclockwise around the length of fiber coil 110 (hereinafter, xe2x80x9cthe ccw wavexe2x80x9d).
After being transmitted from fiber coil 110 to multifunctional integrated optics chip 120 via optic fibers 124 and 126, respectively, the ccw and cw wave are combined at y-junction 132 before being further provided to port c of fiber coupler 116 via optic fiber 118. As noted above, once the two light waves are provided to port c, the waves are then provided to ports a and b, but not provided to port d.
Port b is further connected to photodetector 138 via optic fiber 136 such that the ccw and cw light waves are received at the photodetector 138, which in turn, provides an output photocurrent i to analog signal conditions unit 140. The value of photocurrent i is proportional to the intensity of the two electromagnetic waves or light waves impinging on the photodetector 138. Therefore, the photocurrent i is expected to follow the cosine of the phase difference between the two waves which impinging on the detector 138.
In the prior art arrangement depicted, the output signal from photodetector 138 may be converted to a voltage and amplified at analog signal conditioning unit 140 (ASC). The output voltage signal may then be further provided to an analog to digital converter 142 where it is converted to a digital signal prior to being passed to PSD/digital demodulator 144. PSD/digital demodulator 144, serving as part of a phase demodulation system, is a well known device. Such a PSD/digital demodulator 144 extracts the amplitude of the fundamental frequency fb of the photodetector 138 output signal, or the fundamental frequency of modulation signal generator 146 plus higher odd harmonics, to provide an indication of the relative phase of the electromagnetic waves impinging on photodetector 138. This information is provided by PSD/digital demodulator 144, as the output of the gyroscope.
Typically, gyroscopic designers seeking to minimize excess noise (e.g., xe2x80x9crelative intensity noisexe2x80x9d) employ techniques which seek to phase modulate the light counterpropagating within the fiber coil so that the working point for signal measurement is always in the characteristic range of maximum measuring signal change per rotation rate change. That is, designers seeking to maximize the sensitivity of the gyroscope to sensing angular rotations must consider the maximum modulation which can occur for a particular gyroscope configuration, in order to maximize the gyroscope""s sensitivity. Various conventional relative intensity noise or excess noise suppression techniques are described in U.S. Pat. No. 6,204,921 issued Mar. 20, 2001 to Strandjord et al., and incorporated herein by reference in its entirety.
One type of excess noise reduction technique found in the prior art, called the xe2x80x9csubtractionxe2x80x9d technique, is illustrated with reference to FIG. 2, in which like character references as that of FIG. 1 indicate similar components of similar operation. Unlike what is depicted in FIG. 1, the portion of the light source 112 directed to port d of coupler 116 is utilized. That is, the light signal which is directed to optical fiber portion 134 is further provided to a second photodetector 250, where the signal is converted into a second photocurrent. The second photocurrent generated by photodetector 250 is further provided to a variable gain amplifier 251, where it is amplified prior to being provided to an analog adder 253.
In similar manner, coupler 116 provides a light signal to a first photodetector 138 via port b and fiber optic fiber portion 136. First photodetector 138 then converts the signal into a first photocurrent which is then provided to analog adder 253. Analog adder 253 may be any conventional adder for combining analog signals. Therefore, at analog adder 253, the first photocurrent and second photocurrent are summed to produce a summed photocurrent for providing to ASC 116.
For ideal optical components, the excess noise observed at first and second photodetectors 138 and 250, respectively, is correlated. That is, for a fiber optic gyroscope operating with a bias modulation at the coil eigen frequency, the excess noise at photodetector 138 occurring at the eigen frequency including odd harmonics will be 180 degrees out-of-phase with the noise at photodetector 250 occurring at the same frequencies. Therefore, by adding a properly gain adjusted signal from photodetector 250 to the signal from photodetector 138, the noise at the output of the adder 253 associated with excess noise will be reduced to zero at the eigen frequency and odd harmonics for a gyro employing ideal components. However, imperfections in real optical components such as polarization crosstalk will limit how much the excess noise is actually reduced.
An additional disadvantage of the xe2x80x9csubtractionxe2x80x9d technique is that, in order to have a high level of excess noise reduction, the amplitude adjustment of the signal from photodetector 250 must be relatively accurate. In particular, the amount of amplitude adjustment depends on many system parameters, such as, the responsivity of photodetectors 138 and 250, the gain of the amplifier 251, the bias modulation amplitude and optical loss in the wave propagating path from fiber coupler 116 through multifunction chip 122 and fiber coil 110 and back through the fiber coupler 116 to photodetectors 138 and 250. Moreover, it is important to note that these systems"" error in amplitude adjustment will increase as the system parameters change over time with the aging of the gyro.
The change in system parameters becomes even more pronounced in systems employing high performance fiber optic gyroscopes, such as space applications which are exposed to radiation or submarine navigation applications which encounter an aging mechanism in the coil fiber that causes increased optical loss over time. In those systems, the amplitude adjustment made on the signal from detector 250 must typically be updated in order to track the drift which often readily occurs with regard to the noted system parameters. Consequently, the updating of the parameters is typically done by using a variable gain amplifier where the gain control 252 is adjusted based on a ratiometric measurement of the light detected at the photodetectors 138 and 250. As should be understood, the variable gain amplifier and circuits used to perform the ratiometeric measurements adds undesirable complexity to the design and operation of the gyroscope.
It should be noted, however, that the disadvantages inherent in the xe2x80x9csubtractionxe2x80x9d technique may typically be overcome by implementing an excess noise servo. In general, employing an excess noise servo typically involves providing a portion of the superfluorescent fiber light source to the servo, which, in turn, uses the provided light to control the light source pump current (e.g., negative feedback). In this way, the light output intensity of the superfluorescent fiber light source becomes a function of the pump current, such that, random fluctuations in the intensity of the light output may be cancelled by applying the appropriate changes in pump current levels.
However, where a high performance fiber optic gyroscope used erbium fiber, it was believed that using the subtraction technique with servo control was impracticable. That is, it should be understood that the typical bias modulation frequency fb of conventional high-performance fiber optic gyroscopes may be around 20 kHz to 50 kHz. In addition, one skilled in the art will understand that the fundamental demodulation frequency of a conventional high-performance fiber optic gyroscope may be the same as the bias modulation frequency. As shown by equation (1) below, the demodulator output noise depends on the input noise at the fundamental frequency and odd harmonics. Therefore, a careful inspection of equation (1) reveals that to reduce the effect of excess noise on angle random walk, the excess noise is typically reduced at the demodulation frequency fundamental, 3 rd and 5 th harmonics.                               σ          out                =                              R            f                    ⁢                      G            A                    ⁢          B          ⁢                                                    ∑                                  i                  =                  0                                ∞                            ⁢                              xe2x80x83                            ⁢                                                (                                                                                    G                        f                                            ⁡                                              [                                                                              (                                                                                          2                                ⁢                                i                                                            +                              1                                                        )                                                    ⁢                                                      f                            b                                                                          ]                                                              ⁢                                          1                                                                        2                          ⁢                          i                                                +                        1                                                              ⁢                                                                  i                        n                                            ⁡                                              [                                                                              (                                                                                          2                                ⁢                                i                                                            +                              1                                                        )                                                    ⁢                                                      f                            b                                                                          ]                                                                              )                                2                                                                        (        1        )            
For high performance erbium fiber optic gyroscopes, however, it was believed that reducing the excess noise of the gyroscopic system at the demodulation fundamental, 3 rd and 5 th harmonics was impracticable because the upper state lifetime of the erbium fiber (e.g., erbium atoms) would limit how fast the output light could be controlled. That is, previously, fiber optic gyroscope designers thought that after about 100 hertz, the frequency response of the erbium fiber would be ineffectual for controlling the excess noise output of the gyroscope via the pump current. Moreover, it was believed that the light output of the erbium fiber light source couldn""t be controlled fast enough to manage intensity variations in the 20 kHz to 50 kHz range. Furthermore, the designers believed that the bandwidth within which an excess noise servo could operate in a system using erbium doped optical fiber would be limited to less than 100 Hz.
Recent experimentation on the frequency response of erbium fiber, however, has yielded different and unexpected results. Namely, it was discovered that the frequency response of the erbium fiber after the cutoff frequency was suitable for use in fiber-optic gyroscope technology in that the roll off after the cutoff frequency permitted increased control of the relative intensity noise servo. For example, it was discovered that after the cut-off frequency of the erbium fiber (e.g., the 3 kHz cutoff frequency, when a pump intensity power was provided from the pump at a wavelength of 1480 nm, and this was provided to the erbium fiber which emitted light at a wavelength of 1550 nm), the intensity rolloff of the pump-erbium fiber combination was only 6 db/octave, which made it manageable to construct a stable control loop. If the rolloff was too high, then the control loop would not be useable. Consequently, it was discovered that the unexpected characteristics of the erbium fiber allows construction of a relative intensity noise servo with a bandwidth of at least 100 KHz to 500 kHz. This, in turn, provides a noise reduction realization of a factor of 4 at the output of the fiber optic gyroscope demodulator.
Until now, the use of the properties of the erbium fiber to enhance the gain in the fiber optic system has gone untried because of the erroneous belief that the relatively long upper-state life-time of the erbium atoms would limit any control of light intensity to well below 1 kHz. Hence, a need existed for a system for use in reducing the excess noise of gyroscopic system at the demodulation fundamental, 3 rd and 5 th harmonics which allows additional control of the angle random walk and relative intensity noise by capitalizing on the gain provided the gyroscope by the erbium fiber. Presently known control methods for controlling excess noise gyroscopes using erbium remain inadequate, particularly in their ability to limit excess noise and provide pump current control at low frequencies.
Various embodiments of methods and systems are provided for reducing relative intensity noise in a high performance fiber optic gyroscope, which addresses many of the shortcomings of the prior art. Particularly, various methods and systems are provided for reducing the excess noise present in a erbium-doped fiber optic gyroscope by manipulating the intensity of the light provided by an erbium-doped light source in response to the gain attributable to the gain characteristics of the erbium-doped fiber.
In accordance with various aspects of the system, the present invention provides a system for suppression of relative intensity noise in a fiber optic gyroscope in which the system takes advantage of the frequency response of an erbium fiber to control variations in pump current, and thus control fluctuations in the gyroscopic light source. Particularly, various embodiments use the erbium fiber frequency response to facilitate a stable control loop feed back design for controlling the pump current. More particularly, the invention takes advantage of the recent discovery that the frequency response of erbium fiber above 3 kHz closely mimics an integrator with a 6 dB/octave rolloff, which allows for a relative intensity noise control loop with a bandwidth much greater than 3 kHz. With this type of frequency response, a stable loop with positive gain in the frequency range of 20 kHz to 200 kHz, or higher, is provided.
In accordance with one exemplary aspect, a portion of the erbium fiber light source (e.g., xe2x80x9clight signalxe2x80x9d) is provided to a photodetector for detecting the fluctuations in the light intensity of the erbium light source. The photodetector converts the light signal into an electrical signal prior to the signal being amplified. A constant direct current (dc) signal is impressed upon the amplified signal and then the combined signal is further provided to the current control input of a pump injection current driver, the output of which is the injection current supplied to the pump laser for conversion into optical power. The optical power is then provided to the erbium fiber light source which, in turn, causes the erbium fiber to emit light at a wavelength representative of a nominal intensity level.
In accordance with another aspect of the invention, the fluctuations in light intensity caused by the existence of excess noise or relative intensity noise is reduced via a control loop in which the amount of reduction is a function of the open loop gain of the control loop. The open loop gain of the control loop is enhanced by the additional gain provided to the loop due to the erbium fiber.
In accordance with yet another aspect of the invention, a servo control is provided to facilitate the control of the intensity fluctuations of an erbium-doped light source in response to the gain to the overall system attributable to the erbium-doped light source.